Conventionally, some diesel engines have a selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a feature of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the selective reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of NOx to be discharged.
It has been well known in a field of flue-gas denitration in industrial plant or the like that ammonia (NH3) is effectively used as reducing agent to depurate NOx through reduction. However, it is difficult for automobiles to ensure safety in driving with ammonia itself carried, so that use of nontoxic urea water as reducing agent has been recently researched (see, for example, Patent Literature 1).
More specifically, when urea water is added to the exhaust gas upstream of the selective reduction catalyst, the urea water is decomposed into ammonia and carbon dioxide gas according to the following equation to thereby depurate NOx in the exhaust gas well through reduction by ammonia on the catalyst.(NH2)2CO+H2O→2NH3+CO2 
As shown in FIG. 1, known as the prior art for addition of urea water 3 into exhaust gas 2 upstream of a selective reduction catalyst 1 is a nozzle 4a of a urea water addition unit 4 inserted near an axis of an exhaust passage 5 so as to add the urea water 3 centrally of a flow of the exhaust gas 2, which brings about the nozzle 4a of the addition unit 4 to be directly subjected to the flow of the high-temperature exhaust gas 2, leading to anxiety about a durability of the addition unit 4.
In order to overcome this, as shown in FIG. 2, it has been under review that an injection port 6 slantingly divergent upstream is provided on a side surface of the exhaust passage 5, the urea water addition unit 4 being fitted to the injection port 6 from outside of the exhaust passage 5 to front an inside of the exhaust passage 5, the urea water 3 being added by the addition unit 4 which is protected not to be directly subjected to the flow of the high-temperature exhaust gas 2.